Chemical Weathering Processes

In the previous tutorial, we learned about physical weathering, which breaks down rock via mechanical processes. So now let’s move on to chemical weathering. Chemical weathering is the collection of processes that cause changes to rocks and minerals on the molecular level. As general rule of thumb, minerals with higher crystallization temperatures tend to weather faster at the surface. Chemical weathering almost exclusively involves reactions between minerals and natural, aqueous solutions. There are two types of chemical weathering reactions, congruent and incongruent. Congruent reactions are the simple dissolution of minerals into their constituent ions. Take for example the dissolution of halite, or sodium chloride, which breaks down into Na+ and Cl- in water. By contrast, incongruent reactions produce entirely new minerals or amorphous solids. For example, the sodium feldspar albite reacts with water and carbon dioxide to form kaolinite clay and dissolved sodium ions, bicarbonate ions, and silicic acid, as is shown by this equation here. The most important chemical weathering reactions are acid-base reactions, which involve the exchange of protons, and redox reactions, which involve the exchange of electrons. These should both be very familiar from our study of general chemistry. Before we get into acid-base reactions, we must consider that every drop of rainwater contains some dissolved atmospheric gases, those being nitrogen, oxygen, argon, and most importantly carbon dioxide. In the atmosphere, water reacts with dissolved carbon dioxide to form carbonic acid, or H2CO3, and any water in equilibrium with the atmosphere will have naturally acidic pH of around 5.6. Areas with bad air pollution will have rain that is even more acidic, due the formation of sulfuric acid and nitric acid from the reaction of rainwater with hydrocarbon combustion gases. Furthermore, biological activity in the shallow subsurface releases additional CO2 that gets dissolved in the groundwater, further decreasing its pH to around 4 or 5. As we know from learning about acids and bases, acidic solutions have surplus of hydronium ions. It is these ions that react with minerals to break them down. Recall the chemical weathering of albite, this is an acid-base reaction where it is attacked and broken down by hydrogen ions, causing the release of sodium ions and silicic acid into solution. Since many weathering reactions involve carbon dioxide, bicarbonate ions are common product. As result, bicarbonate is by far the most abundant anion in natural waters. The production of bicarbonate during weathering also acts to neutralize rainwater’s pH as it infiltrates deeper into the ground, causing groundwater to become either neutral or slightly basic. Therefore, most chemical weathering occurs near the surface when groundwater is most acidic and therefore most reactive. The chemical weathering of silicate rocks is the most important long-term mechanism for removing carbon dioxide from the atmosphere via its conversion to bicarbonate and eventual transport to the ocean basins where it reacts with calcium to form calcite, then sinking down to the bottom of the ocean where it is stored for eons. Chemical weathering involving redox reactions is extremely important when transition metals, especially iron, are involved. At the most basic level, chemical weathering occurs because most minerals form deep within the Earth, where the pressure and temperature are orders of magnitude greater than at the surface. Thus, when they are exhumed, they are often unstable and break down into minerals that are more stable under these new conditions. The difference between the pressure and temperature at the surface and deep within the lithosphere is great, between 2 and 4 orders of magnitude, but this is nothing compared to the difference in oxygen concentration, which can be over 15 orders of magnitude. Because of this, compounds that contain metals with multiple oxidation states become oxidized, or in other words, get some of their valence electrons stolen by oxygen. Weathering-related oxidation is extremely common in iron-containing minerals like olivine, pyroxene, and biotite. For example, fayalite, the iron endmember of olivine, is first attacked by acidic waters, breaking it down to silicic acid and dissolved Fe2+ ions. Next, aqueous Fe2+ has one of its electrons stolen by oxygen and becomes Fe3+, which then reacts with water to from precipitates such as amorphous ferric hydroxide, goethite, or FeO(OH), and hematite, or Fe2O3. Compounds of ferric iron are notoriously insoluble and are often responsible for the red color in rocks. Another mineral that weathers via oxidation is pyrite, which contains one Fe2+ ion and two S- ions. In the presence of oxygen-rich waters, both iron and sulfur are oxidized and react with water to form ferric hydroxide and sulfuric acid. The creation of sulfuric acid through this and similar reactions is responsible for destructive acid mine drainage that can damage ecosystems and contaminate drinking water. Acid mine drainage is doubly harmful, because not only is the acidic water harmful, but it also is capable of dissolving large amounts of metals, thereby further contaminating rivers and streams. The characteristic yellow color of acid mine drainage, called yellow boy, is caused by Fe3+ and other metals dropping out of solution and forming precipitates as its pH rises outside of the mine and it is progressively diluted by normal surface water. And with that we have covered some important physical and chemical weathering processes. Let’s move forward and check out some different types of weathering environments, and the rock characteristics that they produce.